Teston is a fine example of translating Photoshop tuts to GIMP, and also the reason I began examining Pyrandon's tut myself. (You've had more success with his tut than I have, but you started this map with it, maybe my next go will be better). I think it turned out well.

Sorry if this comes a little late, as we are in finished maps forum, but bevel and emboss function under layer tab> layer effects with your buildings selected, inner bevel, size large enough to cover your buildings (each 1 = 1 pixel into your buildings from all sides, so half your largest building will do), angle (looks like your sun is at 1:30 so -15 degrees) will give your roofs peaks, shadows and highlights on appropriate sides.

I'd rep ya again in gratitude, but apparently I haven't been generous enough since the last time I repped you.